A known power semiconductor device is the integrated gate commutated thyristor (IGCT) which includes one or more gate commutated thyristor (GCT) cells within a single wafer. Each of the GCT cells is made up from a cathode electrode in form of a cathode metallization layer, an n+-doped cathode layer, a p-doped base layer, an n−-doped drift layer, an n-doped buffer layer, a p+-doped anode layer and an anode electrode in form of an anode metallization layer. The GCT cells also include a gate electrode in the form of a gate metallization layer which is in contact with the doped base layer. The gate metallization layer is arranged in a plane which is below the plane in which the cathode electrodes are arranged in order for the gate metallization layer to be separated from the cathode electrodes. The IGCT includes at least one gate contact in the form of an annular metallic region either in the centre of the wafer, at the perimeter of the wafer or somewhere in between. The gate contact region is in direct contact with the gate metallization layer so that the gate contact region and the gate electrodes of all GCT cells are connected electrically and thermally with each other.
Another known power semiconductor device is the reverse conducting integrated commutated thyristor (RC-IGCT) which includes within one single wafer an IGCT part and a single built-in freewheeling diode part. The freewheeling diode part is made of one single diode including a p-doped anode layer and an n+-doped cathode layer, which are separated by the n−-doped drift layer and the n-doped buffer layer. The diode is arranged adjacent to the IGCT part in such a way as to either be in the innermost or outermost region of the wafer. Between the IGCT part and the freewheeling diode part there exists an n−-doped separation region which separates the p-doped base layers of the GCT cells in the IGCT part from the p-doped anode layer of the diode.
In FIGS. 1 and 2 there is shown a power semiconductor device known as the bi-mode gate commutated thyristor (BGCT). FIG. 1 shows the device in top view and FIG. 2 shows the device in cross section taken along line c′-c in FIG. 1. The BGCT is similar to the RC-IGCT and comprises in a single wafer 1 a plurality of gate commutated thyristor (GCT) cells 2 electrically connected in parallel to one another. The GCT cells 2 in the BGCT are identical to the GCT cells found in the RC-IGCT. In the BGCT shown in FIGS. 1 and 2 each of the GCT cells 2 is made up from three cathode electrodes 3 in form of a cathode metallization layer, an n+-doped cathode layer comprising three strip-shaped cathode segments 4, a p-doped base layer 5, an n−-doped drift layer 6, an n-doped buffer layer 7, a p+-doped anode layer 8 and an anode electrode 9 in form of an anode metallization layer. Like in the IGCT, the GCT cells 2 also include a gate electrode 10 in the form of a gate metallization layer which is in contact with the doped base layer 5. The gate metallization layer is arranged in a plane which is below the plane, in which the cathode electrodes 3 are arranged, so that the gate metallization layer is separated from the cathode electrodes 3. The BGCT includes one single gate contact 11 in the form of an annular metallic region in the centre of the wafer 1. The gate contact 11 is in direct contact with the gate metallization layer so that the gate contact 11 and the gate electrodes 10 of all GCT cells 2 are connected electrically and thermally with each other.
In contrast to the RC-IGCT the BGCT comprises not only a single freewheeling diode part with a single diode but a plurality of diode cells 12 distributed between the GCT cells 2. The diode cells 12 are electrically connected in parallel to one another and to the GCT cells 2, albeit with opposing forward direction. Each diode cell includes an anode electrode 17, a p-doped anode layer 13 and an n+-doped cathode layer 14, and a cathode electrode 16, wherein the p-doped anode layer 13 and the n+-doped cathode layer 14 are separated by the n−-doped drift layer 6 and the n-doped buffer layer 7. Neighbouring GCT cells 2 and diode cells 12 are separated by multiple separation regions 15.
FIG. 3 shows a cross section of a modification of the BGCT shown in FIG. 2, in which in the GCT cell 22 cathode segments 4 that lie in the immediate proximity of a diode cell 12 are surrounded by a gate electrode 20 from all sides. In this case the separation region 15 is laterally spaced from the next cathode segment 4 by a greater distance than in the embodiment shown in FIG. 2 as a wider base layer is required to have enough space for the portion of the gate electrode 20 which is arranged between the separation region 15 and the adjacent cathode segment 4, respectively. Elements in FIG. 3 having reference signs already used in FIG. 2 are identical to the corresponding elements of the device shown in FIG. 2.
The BGCT is disclosed in WO 2012/041958 A2 and in the article “The concept of Bi-mode Gate Commutated Thyristor—A new type of reverse conducting IGCT” by U. Vemulapati et al. in Power Semiconductor Devices and ICs (ISPSD), 2012, pp. 29 to 32, for example.
One of the advantages of the distributed diode and IGCT cells in the same wafer in the BGCT design over the standard RC-IGCT design is the better thermal resistance, because due to the distributed diode cells 12 and IGCT cells 2 the heat is distributed more uniformly in the wafer 1. For example when the BGCT is working in IGCT mode the heat can easily spread from the GCT cells 2 into the diode cells 12. Instead, in the standard RC-IGCTs, the temperature spreads much less efficiently into the diode area because it is concentrated in one continuous area. The same phenomenon is observed when the device is operating in the diode mode.
The maximum controllable current (MCC) and the on-state voltage are important parameters in the above described devices. It is desired to achieve the highest possible MCC and the lowest possible on-state voltage to minimize the losses in the device. Further, uniform turn-on and turn-off are most critical to avoid local overheating.